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The head and neck regions contain many complex anatomical structures. There are numerous vessels, nerves and muscles that are designed to facilitate the many functions of the head and neck.
The neck is the region connecting the head to the trunk. It is commonly described as extending from the base of the skull and the lower margin of the mandible above to the upper border of the clavicles below. It acts as a pathway for structures to enter and leave the head.
Nature has compartmentalised structures using layers of cervical fascia, thin layers of connective tissue that form barriers between structures in the neck. There is one superficial fascia and three deep fascia.
There is a superficial fascial layer just below the skin, housing superficial veins such as the external jugular vein, superficial lymph nodes and the platysma muscle (a thin sheet of muscle under the skin of the neck). On the image below, the superficial layer is not labelled, but would run in the beige coloured soft tissue that surrounds the neck.
The investing layer of the deep cervical fascia is the most superficial of the three. It surrounds the neck like a collar and holds within it the sternocleidomastoid (the large muscle running from behind the ear down to the breastbone and collarbone) and trapezius muscles, and the submandibular and parotid salivary glands. On the image below, this layer is shown in orange.
The pretracheal layer of the deep cervical fascia is the next layer. It surrounds only the anterior and lateral parts of the neck. Its muscular component surrounds the infrahyoids (the muscles below the hyoid bone, a small U-shaped bone at the front of the neck). Its visceral component (the part around the organs) surrounds the thyroid gland, trachea and oesophagus. In the image below, the muscular component is shown in navy blue, and the visceral component is shown in lilac.
The prevertebral layer of the deep cervical fascia forms a sheath for the vertebral column and muscles associated with it. In the image below, it is shown in pink.
The buccopharyngeal fascia is the posterior part of the visceral component of the pretracheal fascia. It lies behind and to the sides of the pharynx and covers its muscles. It allows for the formation of the retropharyngeal space, a deep neck space that sits behind the buccopharyngeal fascia. Infections that develop in this space have the ability to extend into the thorax, potentially causing a life-threatening mediastinitis (infection of the tissues in the centre of the chest).
Diagram - A cross-section through the neck, showing the different fascia of the neck outlined in different colours
SimpleMed original by Dr. Maddie Swannack
The three key superficial muscles of the neck are the platysma, trapezius, and sternocleidomastoid muscles.
Table - Superficial muscles of the neck, their innervation and action
SimpleMed original by Dr. Ethan Kavanagh
The hyoid muscles are situated on the anterior neck and can be split into the suprahyoid and infrahyoid muscles, all of which attach to the hyoid bone in the neck. The four suprahyoid muscles are responsible for elevating the hyoid and depressing the mandible (opening the mouth). The infrahyoid muscles overlie the larynx and thyroid gland, acting to depress and stabilise the hyoid bone.
The scalene muscles (three pairs of muscles running from the neck vertebrae to the upper ribs) are situated on the lateral side of the neck, and this group is made up of the anterior, middle and posterior scalene muscles. They contribute to the flexion of the neck and can be used as accessory muscles to aid in breathing.
Diagram - The muscles and the bones of the neck. The suprahyoid muscles are indicated by the green arrow head, the infrahyoid muscles are indicated by the blue arrow head, and the lateral muscles of the neck are indicated by the orange arrow head
Public Domain Source, edited by Dr. Maddie Swannack [Public domain]
These muscles can be classified based on their physical action - either they can constrict in a circular fashion or pull in a linear fashion.
The facial nerve supplies the muscles of facial expression.
It has five main extra-cranial (arising outside of the skull) branches, namely: temporal, zygomatic (cheekbone), buccal (cheek), marginal mandibular and cervical (remembered by the mnemonic ‘to Zanzibar by motor car’).
It is important to remember that there are two facial nerves, left and right, which each supply respective halves of the face.
The platysma muscle is a very thin, sheet-like muscle that covers the anterior portion of the neck.
It depresses the lower jaw and tenses the skin of the neck.
It is supplied by the cervical branch of the facial nerve.
The orbicularis oculi muscle is the round muscle that surrounds the eye.
It is responsible for closing the eyelids.
It has two main divisions:
- The orbital portion is thicker, and its fibres form an uninterrupted ring around the eye. It is responsible for the forceful closure of the eye and is supplied by the temporal and zygomatic branches of the facial nerve.
- The palpebral portion (the part in the eyelid) is thinner and pale. It contains the preseptal and pretarsal muscles of the eyelid (the parts in front of the orbital septum, a thin membrane in the eyelid, and in front of the tarsal plate, the firm plate that stiffens the eyelid). This portion is responsible for the spontaneous blink and is supplied by the zygomatic branch of the facial nerve.
The occipitofrontalis is a muscle with two parts, called bellies (the fleshy parts of a muscle), joined by the epicranial aponeurosis (described below). The frontal belly lies in the forehead and is responsible for raising the eyebrows. The occipital belly lies over the back of the skull and pulls the scalp backwards.
The frontal belly is supplied by the temporal branch of the facial nerve, and the occipital belly by the posterior auricular branch (a branch that runs behind the ear).
The orbicularis oris muscle encircles the lips.
It is responsible for closing the mouth and causing the lips to pucker, earning it the nickname ‘the kissing muscle’.
It is supplied by the buccal branch of the facial nerve.
The buccinator muscle sits between the mandible and the maxilla, forming the anterior portion of the cheek and the lateral portion of the oral cavity.
It compresses the cheek against the teeth, helping to prevent food accumulation during chewing, and assists with actions such as blowing and sucking.
It is supplied by the buccal branch of the facial nerve.
The levator palpebrae superioris muscle is a muscle contained within the orbit that elevates the upper eyelid.
It is supplied by the oculomotor nerve (cranial nerve III).
If this muscle or the oculomotor nerve is not functioning correctly, the eyelid will droop, causing ptosis.
The epicranial aponeurosis is a dense layer of fibrous tissue that covers the top of the skull.
The scalp has five layers, remembered by the acronym SCALP. The epicranial aponeurosis is one of these layers. The five layers are: Skin, Connective tissue, Aponeurosis of the Occipitofrontalis muscle, Loose connective tissue, and the Pericranium of the skull (the periosteum, the membrane covering the bone).
Diagram - The front and side views of the muscles of facial expression
Creative commons source by CNX Anatomy 2013 [CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0)]
Compared to the numerous muscles of facial expression, there are only four muscles responsible for mastication, the elevation and depression of the mandible (lower jaw), along with its forward, backward and side-to-side movements, to allow the chewing of food.
The trigeminal nerve supplies the muscles of mastication (the muscles responsible for chewing), as well as providing sensory innervation to the face through three branches, namely the ophthalmic, maxillary and mandibular branches. For more information on the trigeminal nerve, please see the article on the first six cranial nerves found here.
The four muscles of mastication are all innervated by a motor branch of the trigeminal nerve and they all act on the temporomandibular joint (TMJ, the jaw joint). These muscles are:
- Temporalis
- Masseter
- Medial Pterygoid
- Lateral Pterygoid
The temporalis, masseter and medial pterygoids are all responsible for the elevation of the mandible, and together provide enough force to chew tough food.
The lateral pterygoid contributes to depression of the mandible. Jaw opening also involves gravity and the suprahyoid muscles when the hyoid bone is held steady by the infrahyoid muscles.
The lateral pterygoid cannot be palpated clinically, while the medial pterygoid can be palpated intraorally (from inside the mouth). Place your fingers over your temples (for temporalis) or over the corner of your jaw (for masseter) and clench. You will feel the muscles contracting.
Structures that run between the head and the thorax are associated with the anterior triangle.
These structures include:
- Common carotid artery that bifurcates within the triangle into the internal and external carotid arteries.
- Internal jugular vein
- Cranial nerves - facial (VII), glossopharyngeal (IX, supplying the throat and the back of the tongue), vagus (X), accessory (XI) and hypoglossal (XII, supplying the tongue muscles).
The borders of the anterior triangle are:
- Lateral - anterior border of the sternocleidomastoid muscle.
- Medial - imaginary sagittal line down the midline of the neck.
- Superior - lower border of the mandible.
The anterior triangle of the neck is subdivided into four smaller triangles: the submental (under the chin), submandibular (under the jaw), carotid and muscular triangles. The carotid triangle is described below.
The carotid triangle is a region of the anterior triangle (mentioned above). It has the following boundaries:
- Superior - posterior belly of digastric muscle (a muscle under the jaw with two bellies).
- Lateral - anterior border of sternocleidomastoid muscle.
- Inferior - superior belly of omohyoid muscle (a thin strap-like muscle running from the hyoid bone to the shoulder blade).
The main contents of the carotid triangle are:
- Common carotid artery - which bifurcates within the triangle into the external and internal carotid arteries.
- Internal jugular vein
- Hypoglossal nerve
- Vagus nerve
Structures in the carotid triangle are quite superficial, making this triangle a frequent access point for surgical intervention.
Diagram - The anterior triangle in blue and the carotid triangle in pink. It shows the carotid triangle as a subdivision of the anterior triangle
Creative commons source by Olek Remesz, edited by Dr. Maddie Swannack [CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0)]
Posterior Triangle of the Neck
Structures that run between the thorax and upper limb are associated with the posterior triangle.
These structures are:
- External jugular vein
- Subclavian artery (its third part, which runs from the lateral edge of the anterior scalene muscle to the outer border of the first rib) and the subclavian vein, which lies in front of the artery and at a slightly lower level
- The accessory nerve (cranial nerve XI)
- Cervical plexus, including the phrenic nerve
- Trunks of the brachial plexus
The borders of the posterior triangle are:
- Anterior - posterior border of sternocleidomastoid muscle
- Posterior - anterior border of trapezius muscle
- Inferior - middle third of the clavicle
Structures of the posterior triangle are important surgically:
- The subclavian vein can be used as a point of access to the venous system via a central catheter (a long thin tube that ends in a large vein near the heart). Pneumothorax (air in the space around the lung) is a recognised complication and is most common with the subclavian route. Ultrasound guidance helps to reduce this risk, and the internal jugular vein may be chosen instead because it may carry a lower risk of mechanical complications.
- The cervical plexus can be numbed with an injection of local anaesthetic (a cervical plexus block). The injection is given midway up the posterior border of the sternocleidomastoid, where the skin branches of the plexus emerge (the nerve point of the neck). It is used for neck surgery such as thyroidectomy (removal of the thyroid gland) and carotid endarterectomy (clearing a narrowed carotid artery), and care is needed because it can affect the phrenic nerve.
Diagram - The posterior triangle of the neck in green
Creative commons source by Olek Remesz, edited by Dr. Maddie Swannack [CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0)]
Edited by: Dr. Maddie Swannack
Reviewed by: Dr. Thomas Burnell and Dr. Marcus Judge
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